Steam generating unit with corner fired furnace and gas recirculation



Nov. 18, 1958 I w. HELLER ETAL 2,860,613

STEAM GENERATING UNIT WITH CORNER FIRED FURNACE AND GAS RECIRCULATION Tlc lo.

Filed April 30, 1953 2 Sheets-Sheet l ATTORNEY L. W. HELLER E ATIN ET AL 2,869,613 STEAM GEN R G UNIT WITH CORNER FIRED FURNACE AND GAS RECIRCULATION Filed April 30, 1953 Fig.2.

' ATTORNEY United States Patent STEAM GENERATING UNIT WITH CORNER FIRED FURNACE AND GAS RECHRQULATION Lewis W. Heller, Yardley, Pa., and Charles 81. Smith, Westfield, N. 1., assignors to The Babcock & Wilcox Company, New York, N. Y., a corporation of New Jersey Application April 30, 1953, Serial No. 352,228 12 Claims. (Cl. 122-478) This invention relates to the construction and operation of vapor generators and more particularly, of fuel burning steam generating and superheating units operating at high steam pressures and temperatures.

The invention is particularly directed to apparatus for and a method of varying the radiant transmission of heat to fluid heating tubes positioned in the furnace walls of a unit of the character described, by the recirculation to the furnace of relatively cool gaseous products of combustion withdrawn from a position beyond convection heat absorbing surface. The invention is more particularly directed to a special arrangement for introducing the recirculated flue gases for the described purpose to a furnace in which the fuel is supplied by so-called corner tangential firing burners.

While the invention may be advantageously utilized for affecting the rate of heat transfer to furnace wall cooling tubes for superheat temperature control, it may be separately or concurrently used when burning slagforming fuel to modify the temperature of the gases flowing from the furnace outlet to convection heat absorbing surfaces for the purpose of avoiding or mitigating slag accumulation on such surfaces.

It is well known that furnace gas temperatures and the rate of radiant transfer of heat from furnace gases can be regulated by the introduction of recirculated partially cooled products of combustion into the furnace. It is also known that to introduce such gases into the furnace admixed with the air supplied to burn the fuel, so diluting the oxygen content with inertgases that a retardation of combustion is to be expected. This is particularly true when the fuel burners used, particularly for burning a fuel like pulverized coal, are of a long flame type as compared with short flame type burners providing an intense turbulence between the combustion air and the air-borne fuel by the particular construction of the burners. If the quantity of recirculated gas admixed with the secondary combustion air and introduced through the burner ports is a high percentage of the air, dilution may be such that it may be difficult, if not impossible, to maintain ignition of the pulverized fuelprimary air streams. t i

In accordance with the invention the recirculated gases are introduced into the furnace in proximity to the points of introduction of the fluid fuel and combustion air through burner assemblies in the corners of a tangentially fired polygonal furnace in such a manner that the radiant heat transmission is reduced without hazarding ignition or unduly delaying combustion. The consequent reduction of heat absorption by the fluid cooled furnace walls of a steam generating and superheating unit permits a greater amount of heat to be carried out of the furnace to the ice 2 convection heat absorbing surfaces, such as the steani superheater, and can be utilized in effecting a regulation of the convection superheating. On the other hand, when recirculated gas is introduced to the furnace and the furnace is operated at high rates of heat liberation, a reduction of the furnace outlet temperature of the combined newly developed products of combustion and the recirculated gases is to be expected. Such a reduction in gas temperature under such heat release rates is advantageous where it is desired to limit the furnace exit gas temperature in order to reduce slagging dificulties with slag-forming fuels, such as pulverized coal.

The Kreisinger et al. Patent 2,263,875, Nov. 28, 1944, Kruger Patent 2,243,909, June 3, 1941, and an article by Elno M. Powell entitled Tilting Burners Provide Flexible Furnace Performance in the magazine Combustion of June, 1945, pp. 36 39, disclosed symmetrical arrangements of variable direction burners positioned at the corners of a polygonal section furnace for firing a slag-forming fuel, such as pulverized coal, in directions substantially tangential to an imaginary circle in the furnace. The invention is applicable to fuel fired furnaces of the character described having fixed corner burners, as well as to such furnaces having horizontally movable burners, such as illustrated in the Kruger patent, or vertically tiltable burners, such as illustrated in the Kreisinger patent and Powell article.

The various'features of novelty which characterized the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, refer: ence should be had to the accompanying drawings and descriptive matter in which we have illustrated and described preferred embodiments of the invention.

Of the drawings:

Fig. l is a sectional elevation of a steam generating and superheating unit embodying the invention;

Fig. la is a fragmentary sectional elevation of a similar unit with a modified form of furnace bottom;

Fig. 2 is a partly diagrammatichorizontal view taken on the line 2-2 of Fig. l; and

Pig. 3 is a fragmentary horizontal view on an enlarged scale of one corner of the furnace shown in Fig. 2.

in the construction illustrated in Figs. 1, 2 and 3,

a vertically elongated furnace chamber 10 of square cross-section is defined by a vertical frontwall 12, op-

posite side walls 14 and 16, and a rear wall 18. The

furnace chamber is designed for dry ash operation, having a hopper bottom 28. A nose bafile 22 extends inward ly from the upper part of the rear wall 18 and unites the furnace side walls and roof 24 to define alateral outlet gas pass 26 which opens into a downfiow gas respectively, and tubes 36 along the side walls are connected. The lower ends of the above mentioned tubes are connected to headers 38, 4t], and 42 respectively and these headers are connected to the water space of the steam and. water drum by suitable downcomer pipes (not shown) to supply water for upward flow through the f heated furnace wall tubes. The tubes lining the front and rear walls have their lower portions converging to define the hopper bottom 20, from which a central transverse ash discharge opening 44 opens to a subjacent ash collecting pit (not shown). The furnace lateral gas outlet is screened by extensions of the rear wall tubes 34. The downflow gas pass 28 is connected to one side of a rotary regenerative type air heater 46, the opposite side of which is connected to a gas outlet duct 48.

Saturated steam from the steam space of the drum 3% is conducted by a transverse row of tubes 5%, which extend along the rear wall of the downflow gas pass to a superheater inlet header 52. A counter-flow primary superheater section formed by vertically spaced 54 of laterally spaced multiple-loop tubes, is positioned in the upper portion of the gas pass 28 and discharges to a transverse steam outlet header 56. The outlet header 56 is connected 'to a conduit 58, which includes a spray attemperator 60, with an inlet header 66 of a pendant type parallel flow secondary superheater section 64 positioned in the lateral gas pass 26 adjacent the furnace outlet. The secondary superheater includes laterally spaced banks of serially connected multiple-loop tubes, the inlet ends of which are connected to the header 6d and the outlet ends to an outlet header 68 from which the highly superheated steam is conducted to a point of use. Vertically spaced serially connected banks 69 of steam reheater tubes are arranged across the downflow pass downstream of the primary superheater section 54.

As indicated in Fig. l, pulverized fuel is supplied to the burners by four pulverizers 70, each pulverizer having two valve controlled discharge conduit connections 72, each supplying a stream of pulverized coal and primary air to the nozzle of a pulverized coal burner, as hereinafter described. The furnace of Fig. 1 is provided with two vertically spaced sets of pulverized fuel burners 74a, 741), each set comprising four burners positioned at the corners of the furnace as shown by Fig. 2. The two pulverized coal and primary air conduits 72 from each pulverizer 70 are arranged to supply fuel to corresponding diagonally opposite burners 7411 or 74b at the same level. The valves 76 in these conduits provide a selective control of the flow through the individual discharge conduits. Secondary air for the combustion of the air-borne pulverized fuel from the pulverizers is supplied under pressure by a forced draft fan 78 which discharges into the air heater as as indicated. Preheated secondary air from the air heater is conducted under pressure by a duct 80 and branch ducts 81 and 83 controlled by dampers 82 to the burner ports 84.

With the operation of the pulverizers receiving raw coal and carrier air and directing streams of primary air and entrained pulverized coal through the conduits 72 .connected to the burner nozzles of one or both sets of burners, and with the flow of preheated secondary air through the secondary air duct 8% and through the burner ports 84 surrounding the burner fuel nozzles 74a and 74b, as shown by Fig. 3, mixing of the fuel and carrier air stream with the preheated secondary air is attained to a sufficient degree that ignition of the fuel is readily ob tained and the fuel burned as it is carried along the flow path towards the furnace chamber gas outlet.

No attempt has been made to illustrate the details of construction and mounting of the fuel burner nozzles 74a and 74b insofar as the introduction of fuel and combustionair to the furnace is concerned, as the prior art mentioned adequately discloses the manner in which such burner nozzles would be constructed and arranged to satisfactorily introduce fuel and air into a furnace chamber from corner points in directions substantially tangential to an imaginary circle in the furnace chamber for creating a clockwise rotating burning mass of combustible elements within the furnace chamber. Either or both sets of fuel burners are constructed in accordance with the disclosures of the mentioned prior art, so that the direction of the fuel and air streams can be adjusted or altered with respect tothe horizontal or in relation to the vertical fur- 4t nace walls to vary the position of the burning fuel mass in the furnace chamber.

In accordance with the invention and as shown in Figs. 1, 2 and 3, relatively .cool heating gases are Withdrawn from a position in the heating gas flow path downstream of the steam reheating tube banks 69. A recirculating gas fan as having a damper controlled inlet duct 88 connected to the downflow gas pass 28 at a position downstream of the reheater and ahead of the air heater 46 is provided. As indicated diagrammatically in Figs. 1 and 2, the fan discharges recirculated gas under pressure through damper controlled ducts 9t) and branch ducts 92 to an inlet chamber 94 having an individual damper 96 and each of which is associated with a corresponding burner fuel nozzle 74a 01' 74!).

As shown in Fig. 3, a narrow vertically elongated recirculated gas discharge port 98 is arranged alongside each burner port 84 to direct a stream of recirculated gas under pressure from the inlet chamber 94 between the position of entry of the secondary air and the plane of the furnace chamber wall on the same side as the axis of the fuel stream relative to the center of the furnace chamber. Certain tubes 3401 are bent out of the plane of the tube row 34 on the adjacent wall for a portion of their length embracing the height of the adjacent burner port 84, so that the recirculated gases will enter the furnace chamber between such tube lengths and the tubes of the adjacent wall. The recirculated gas entering the furnace through the port or nozzle hi5 is initially directed in a stream generally parallel to the adjacent wall and at an acute angle with respect to the direction of discharge of the secondary air and fuel introduction through the burner port, by correspondingly curving the outer surface of a refractory filler ltltl at the outer side of the tubes 34a, as. shown in Fig. 3.

The direction of rotation of the burning fuel and gas mass resulting from the entrance direction and velocities of the fuel and combustion air streams Will be clockwise with the burner arrangement shown in Fig. 2, and the re circulated gas streams are discharged in a corresponding angular relation, so that a layer or lamina of recirculated gas will be maintained between each furnace chamber wall and the newly developed gases of combustion. While the general rotation effect will eventually cause gradual diffusion of the recirculated gases into the fresh combustion gases, the initial portion of the path of the recirculated gases in the furnace is so directed as to maintain a blanketing layer of low temperature gas over a substantial portion of the furnace wall tubes, thereby reducing radiant heat transfer to those tube portions. Each recirculated gas port 98 is supplied with recirculated gas by its individual chamber 94 subject to the corresponding damper controllie. By this arrangement the recirculated gas introduction through the individual ports may be properly adjusted or calibrated, and when desirable the recirculated gas flow to any of the gas ports associated with either or both sets of burners may be completelyshut off.

i In a pulverized fuel fired installation with a hopper type furnace bottom as shown in Fig. l, regulation of convection superheating over a relatively wide load range can be effected by' manipulation of movable burners as disclosed by the above mentioned prior art to vary the direction of the introduction of the streams of fuel and combustion air into the furnace. When the burners are vertically adjustable as disclosed by. Kreisinger et al. 2,363,875, there are several ways in which the recirculation of gases through selected ports 98 will be advantageous. For example, when such a unit is operated at high rates of liberation with the burners downwardly tilted to cause the whirling mass of burning fuel and hot products of combustion to be at a lower position in the furnace chamber, the attainment of a gas tempering effect, i. e. a reduction in furnace outlet gas temperature, for slag control is readily obtainable. For such open a'tion recirculated gas may be introduced horizontally through the ports 98 alongside both sets of downwardly tilted burners, or may be selectively introduced through the ports 98 contiguous to the upper set of burners only. With the last mentioned method, a minimum disturbance to the combustion conditions in the subjacent portion of the furnace chamber will be experienced, yet the recirculated gases flowing from the gas ports at the level of the upper set of burners will be entrained in the gases flowing to the furnace gas outlet and reduce the temper ature of those gases to a safe level.

When thefurnace of Fig. 1 is operated with similar vertically adjustable burners, tilted upwardly to give an elevated flame position in the furnace, as is customary when the unit is operated at fractional loads to raise the superheat temperature, a major portion, if not all, of the recirculated gas is introduced through the ports associated with the lower set of burners. This causes the cooler recirculated gases to fill the lower portion of the furnace, reducing the absorption of heat by the exposed radiant heat absorbing surfaces about that zone, whereby the heat content of the gases leaving the furnace and flowing to the convection superheater will be greater than without gas recirculation.

Alternatively if the unit of Fig. l is operated at a fractional load with the fuel supplied by the pulverizers 70 connected only to the upper set of burners, all or a major portion of the recirculated gas can be introduced through the gas ports contiguous to the idle lower set of burners. Such operation is very efiective in reducing furnace Wall radiant heat absorption in the lower portion of the furnace while avoiding adverse effects on the combustion of the fuel and air from the upper burners. Such introduction of the cool gases to the lower portion of the furnace also lowers the temperature of the gases adjacent the idler burners so that the possibility of these fuel burners is minimized.

The modified steam generating unit shown in Fig. la, has a horizontally extending furnace floor 12d spaced relatively closely below the elevation of the lower set of burners 74a. The floor and the adjacent side walls of the furnace are suitably constructed to collect and retain slag in a molten condition for its periodic discharge to slag disposal apparatus, all of which is well known in the art. The furnace bottom of Fig. 1a, when employed in combination with the boiler, fuel, air and recirculated gas equipment of Figs. 1-3, is so positioned below the level of the lower set of fuel burners 74a, that the heat from the fuel introduced through these burners will be released in such proximity that fluidity of the slag accumulated on. the floor can be maintained at the desired fractional load heat liberation.

In the slagging, type of furnace shown furnace temperatures are always high, and, particularly where the unit is operated with a high furnace outlet gas temperature in order to attain a high convection superheat temperature, there will be a greater problem of overheating in Fig. la, the

regulating combustion and furnace heat absorption condi tions in a manner to avoid or mitigate slagging of the convection heat absorbing surface at and beyond the furnace gas outlet. With the described arrangement of the recirculated gas ports 93, the unit is operable to in troduce recirculated gas at two levels at positions contiguous to the two sets of burner ports. When it is advantageous to reduce the quantity of recirculated gas, the arrangement of control dampers permits selective control with respect to the two sets of burners, or in respect to the two sets of recirculated whether one or both sets of fuel burners are in operation. For example, at a fractional load. it may be desirable to concentrate the heat liberation in the zone adjacent the floor in order to insure the fluidity of the slag in that zone. Under such circumstances if recirculated gas is.introduced for the purpose of increasing the gas mass and heat flowlfi'om the furnace, for example, the

gas ports irrespective of 6 predominant portion, if not all, of the recirculated gases will be introduced through the gas ports at the upper level. Concurrently the fuel input may be predominantly, if not entirely, through the set of burners at the lower elevation.

While in accordance with the provisions of the statutes we have illustrated and described herein the best forms of the invention now known to us, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of theinvention covered by the claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of other features.

What is claimed is:

1. A vapor generating and superheating unit comprising walls defining a furnace chamber having a gas exit opening in the upper part thereof, vapor generating tubes lining the walls of said furnace chamber, a convection type vapor superheater arranged to be heated by gases from said gasexit opening, means for firing said furnace chamber comprising a group of fuel burners symmetrically arranged in the walls of said furnace chamber to discharge fuel and air streams in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace for creating a turbulent rotating burning gas mass in the lower portion of the furnace chamber transmitting heat to the vapor generating wall tubes, and means for withdrawing gaseous products of combustion from a point in the gas flow path downstream of said superheater and introducing the withdrawn gases into said furnace chamber in horizontally directed streams separate from said fuel and air streams at points alongside corresponding fuel burners and in a direction diverging from the axis of discharge of said burner, to maintain a layer of recirculated gas between each burning fuel stream and the more closely adjacent furnace chamber wall.

2. A vapor generating and superheating unit comprising walls defining a furnace chamber having a gas exit opening in the upper part thereof, vapor generating tubes lining the walls of said furnace chamber, a convection type vapor superheater arranged to be heated by gases from said gas exit opening, means for firing said furnace chamber over a wide range of operating loads comprising upper and lower vertically spaced groups of fuel burners, the burners of each of said groups being independently operable and symmetrically arranged in the walls of said furnace chamber to discharge in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace for creating a turbulent rotating burning gas mass in the lower portion of the furnace chamber transmitting heat to the vapor gencrating wall tubes, and means for withdrawing gaseous products of combustion from a point in the gas flow path downstream of said superheater and introducing the withdrawn gases into said furnace chamber at points alongside corresponding burners and diverging in direction relative to the directions of corresponding fuel streams, said points of introduction of recirculated gases being disposed between adjacent furnace wall vapor generating tubes and the adjacent burner whereby a layer of recirculated gas is maintained between each burning fuel stream and the adjacent furnace chamber wall.

3. A vapor generating and superheating unit comprising walls defining a furnace chamber having a gas exit opening in the upper part thereof, vapor generating tubes lining the walls of said furnace chamber, a convection type vapor superheater arranged to be heated by gases from said gas exit opening, means for firing said furnace chamber over a wide range of operating loads comprising upper and lower vertically spaced groups of fuel burners, the burners of each of said groups being symmetrically arranged in the walls of said furnace chamber to discharge fuel and air streams in directions substantially tangentialto an imaginary vertical cylinder located centrally in the furnace for creating a turbulent rotating burning gas mass in the lower portion of the furnace chamber transmitting heat to the vapor generating wall tubes, and means for withdrawing gaseous prodnets of combustion from a point in the gas flow path downstream of said superheater and introducing the withdrawn gases into said furnace chamber in horizontally directed streams separate from said fuel and air streams at points alongside corresponding fuel burners and at the same side thereof as the axis of discharge of said burner is relative to the center of said imaginary circle, to maintain a layer of recirculated gas between each burning fuel stream and the more closely adjacent furnace chamber wall.

4. A vapor generating and superheating unit comprising walls defining a furnace chamber having a gas exit opening in the upper part thereof, vapor generating tubes lining the walls of said furnace chamber, a convection type vapor superheater arranged to be heated by gases from said gas exit opening, means for firing said furnace chamber over a wide range of operating loads comprising upper and lower vertically spaced groups of fuel burners, the burners of each of said groups being symmetrically arranged in the corners of said furnace chamher to discharge fuel and air streams in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace for creating a turbulent rotating burning gas mass in a Zone located in the lower portion of the furnace chamber and transmitting heat to the vapor generating wall tubes, means for independently supplying fuel to said upper and lower burner groups, means defining a recirculated gas port opening to said furnace chamber alongside each of said fuel burners and in a direction diverging from the direction of discharge of the corresponding burner, dampers controlling the flow of gas through said recirculated gas ports, and means for withdrawing gaseous products of combustion from a point in the gas flow path downstream of said superheater and introducing the withdrawn gases as separate streams into said furnace chamber through said recirculated gas ports in a direction consonant with the movement of said gas mass to maintain a layer of recirculated gas between each burning fuel stream and the adjacent furnace chamber wall.

5. In a vapor generating and superheating unit comprising walls defining a furnace chamber having a gas exit opening in the upper part thereof, vapor generating tubes lining the walls of said furnace chamber, a convection type vapor superheater arranged to be heated by gases from said gas exit opening, and means for firing said furnace chamber over a wide range of operating loads comprising upper and lower vertically spaced groups of fuel burners, the burners of each of said groups being symmetrically arranged in the walls of said furnace chamber to discharge fuel and air streams in directions L silbstantially tangential to an imaginary vertical cylinder located centrally in the furnace for creating a turbulent rotating burning gas mass in the lower portion of the furnace chamber transmitting heat to the vapor generating wall tubes therein, the method of regulating the heat content of the gases at said gas exit opening which comprises withdrawing gaseous products of combustion from a'point in the gas flow path downstream of said superheater and introducing the withdrawn gases into said furnace chamber in horizontally directed streams sep- =31 arate from the fuel and air streams at points alongside corresponding fuel burners and in a direction diverging from the axis of discharge of said burner, to maintain a layer of recirculated gas between each burning fuel stream and the more closely adjacent furnace chamber wall.

6 In a vapor generating and superheating unit comprisingwalls defining a furnace chamber having a gas exit opening in the upper part thereof, vapor generating tub es lining the Walls of said furnace chamber, a con- 5 vection type vapor superheater arranged to be heated by gases from said gas exit opening, and means for fir: ing said furnace chamber over a wide range of operating loads comprising upper and lower vertically spaced groups of fuel burners, the burners of each of said groups being symmetrically arranged in the walls of said furnace chamber to discharge fuel and air streams in directions substantially tangential to an imag-inary vertical cylinder located centrally in the furnace for creating a turbulent rotating burning gas mass in the lower portion of the furnace chamber transmitting heat to the vapor generating wall tubes therein, the method of increasing the heat content of the gases at said gas exit opening at fractional operating loads which comprises introducing most of the fuel to be burned through said upper group of fuel burners, and withdrawing gaseous prodnets of combustion from a point in the gas flow path downstream of said superheater and introducing the withdrawn gases in substantially horizontally directed streams separate from the fuel and air streams into said furnace chamber solely at points alongside corresponding burners in said lower group and adjacent thereto to maintain a layer of recirculated gas over a substantial portion of said furnace chamber wall.

7. In a vapor generating and superheating unit comprising walls defining a furnace chamber having a gas exit opening in the upper part thereof, vapor generating tubes lining the walls of said furnace chamber, a convection type vapor superheater arranged to be heated by gases from said gas exit opening, and means for firing said furnace chamber at a position remote from said gas exit with a slag-forming fuel at temperatures above the slag-fusion temperature comprising upper and lower vertically spaced groups of fuel burners, the burners of each of said groups being symmetrically arranged in the walls of said furnace chamber to discharge fuel and air streams in directions substantially tangential to an imaginary vertical cylinder located centrally in the furnace for creating a turbulent rotating burning gass mass in the lower portion of the furnace chamber transmitting heat to the vapor generating wall tubes therein, the method of reducing the temperature of the gases at said gas exit opening which comprises introducing most of the slag-forming fuel to be burned through said lower group of burners, withdrawing gaseous products of combustion from a point in the gas flow path downstream of said superheater and introducing the withdrawn gases in substantially horizontally directed streams separate from the fuel and air streams 'into said furnace chamber in a direction consonant with the movement of said gas mass solely at points alongside corresponding burnersin said upper group and adjacent the furnace chamber wall, whereby streams of recirculated gas are distributed above the rotating burning gas mass, and withdrawing slag in a molten condition from the bottom of said furnace chamber.

8. In a vapor generating and superheating unithaving upright walls defining a furnace chamber with a gas exit opening at its upper part, vapor generating tubes lining the walls of the furnace chamber, a convection type vapor superheater arranged to be heated bythe gases from the furnace chamber, fuel burning means for firing the furnace chamber at positions remote from the gas exit, said means including upper and lowervertically spaced groups of fuel burners symmetrically arranged in the corners of the furnace chamber and normally discharging burning fuel into the chamber in directions substantially tangential to an imaginary upright cylinder located centrally of the furnace for creating a turbulent rotating burning gas mass in a zone located at the lower part of the furnace chamber, the vapor generating wall tubes receiving heat radiantly transmitted from said rotating gas mass, means for independently supplying fuel to the upper and lower burner groups, and gas recirculation system including afan and fan in let ductwork communicating with gas flow at a position downstream in a gas flow sense from the superheater and having fan outlet ductwork with branches leading to positions adjacent the separate fuel burners, said gas recirculation system also havingrecirculating gas directing means associated with each of said branches at individual burners and constructed and arranged to direct the furnace entering recirculated gases in a direction divergent with respect to the direction of the burner efllux and over the inner face of the furnace chamber wall presented by the vapor generating tubes whereby a layer of recirculated gas is maintained between the individual streams of burning fuel and the adjacent furnace chamber wall portions, the directions of introduction of the recirculated gases into the furnace chamber being also consonant with the movement of said rotating gas mass.

9. In a vapor generating and superheating unit having upright walls defining a furnace chamber with a gas exit opening at its upper part, vapor generating tubes lining the walls of the furnace chamber, a convection type vapor superheater arranged to be heated by the gases from the furnace chamber, fuel burning means for firing the furnace chamber at positions remote from the gas exit, said means including upper and lower vertically spaced groups of vertically tilting fuel burners symmetrically arranged in the corners of the furnace chamber and normally discharging burning fuel into the chamber in directions substantially tangential to an imaginary upright cylinder located centrally of the furnace for creating a turbulent rotating burning gas mass in a zone located at the lower part of the furnace chamber, the vapor generating wall tubes receiving heat radiantly transmitted from said rotating gas mass, means for independently supplying fuel to the upper and lower burner groups, and a gas recirculation system including a fan and fan inlet ductwork communicating with gas flow at a position downstream in a gas flow sense from the superheater and having fan outlet ductwork with branches leading to positions adjacent the separate fuel burners, said gas recirculation system also having a recirculating gas directing nozzle associated with each of said branches at individual burners and constructed and arranged to direct upright strata of the furnace entering recirculated gases in a direction divergent with respect to the direction of the burner effiux and over the inner face of the furnace chamber wall presented by the vapor generating tubes whereby a layer of recirculated gas is maintained between the individual streams of burning fuel and the adjacent furnace chamber wall portions, the nozzles being disposed radially outward of said tangential directions and directed in consonance with the direction of movement of the pertinent parts of the rotating gas mass.

10. In a method of generating vapor and superheating the generated vapor to a predetermined temperature over a wide range of rate of steam generation, effecting combustion of fuel to develop high temperature combustion gases, radiantly transmitting heat from said gases to confined streams of a vaporizable liquid to generated vapor therein, superheating the generated vapor by the convection transmission of heat from said gases to spaced apart and confined streams of the generated vapor, the combustion being effected by the discharge of fuel and air streams from a plurality of positions symmetrically distributed in a group embracing the combustion zone each of two zones of different degrees of remoteness relative to the superheating zone in a gas fiow sense, the fuel and air streams of each zone being discharged in directions substantially tangential to an imaginary cylindrical area within the combustion zone for creating a turbulent rotating burning gas mass in the combustion zone, withdrawing combustion gases from a point in the gas flow path downstream of the superheating zone, introducing the withdrawn gases as separate streams separate from the fuel and air introduction with each stream introduced adjacent a separate stream of fuel and air, the separate streams of withdrawn gases being also directed in a stratum or layel interposed relative to a plurality of the enclosed streams of vaporizable liquid and a closely adjacent stream of burning fuel, the streams of recirculated gases being also introduced in directions in consonance with the direction of the fuel streams as substantially tangential. to the aboveindicated imaginary cylindrical area, selectively burning a predominant proportion of the total fuel in either one fuel burning zone or the other under different conditions, and limiting the predominant proportion of introduction of withdrawn gases to positions adjacent one fuel burning zone, or the other.

11. In a method of generating vapor and superheatingthe generated vapor to a predetermined temperature over a Wide range of rate of steam generation, effecting combustion of a slag forming fuel to develop high temperature combustion gases, radiantly transmitting heat from said gases to confined streams of a vaporizable liquid to generate vapor therein, superheating the generated vapor by the convection transmission of heat from said gases to spaced apart and confined streams of the generated vapor, the combustion being effected by the discharge of fuel and air streams from a plurality of positions symmetrically distributed in a group embracing the combustion zone in each of two vertically spaced zones of different degrees of remoteness relative to the superheating zone in a gas flow sense, the fuel and air streams being discharged in directions substantially tangential in each zone to an imaginary cylindrical area within the combustion zone for creating a turbulent rotating burning gas mass in the combustion zone, withdrawing combustion gases from a point in the gas flow path downstream of the superheating zone, introducing the withdrawn gases as separate streams separate from the fuel and air introduction with each stream introduced alongside a separate stream of fuel and air, each of the separate streams of withdrawn gases being directed divergently relative to the efflux of its associated burner and in a stratum or layer interposed relative to a plurality of the enclosed streams of vaporiz-able liquid and the closely adjacent stream of burning fuel, the streams of recirculated gases being also introduced in directions substantially tangential to the above indicated imaginary cylindrical area, and selectively burning a predominant proportion of the total fuel in either one or the other of the spaced fuel burning zones.

12. In a vapor generating and superheating unit, a furnace having vapor generating tubes along its walls, widely vertically spaced horizontal rows of fuel burners for firing the furnace at positions of different degrees of remoteness from the gas outlet of the furnace, the burners in each row being symmetrically distributed at circumferentially spaced positions around the furnace with all of the burners so directed as to discharge fuel and air streams in directions substantially tangential to an imaginary cylinder disposed centrally of the furnace for creating a turbulent rotating burning gas mass in the furnace radiantly transmitting heat to the vapor generating wall tubes, a vapor superheater receiving heat from the furnace gases and including spaced tubes, and gas recycling means including a fan and fan inlet ductwork communicating with the flow of furnace gases downstream of the superheater in a gas flow sense for withdrawing a controlled percentage of the gases after loss of heat in superheating, the gas recycling system also having fan outlet ductwork and nozzles or passage means for direct.- ing the withdrawn gases into' the furnace at a plurality of positions each of which is adjacent one of the burners, each nozzle being disposed between a burner and an adjacent furnace wall and constructed to direct a stratum of the higher density recycled gases along the Wall. to blanket the same between the wall and the efflux of the adjacent and associated burner, the outlet nozzles of the gas recycling system being so disposed and directed that the efilux of recycled gases therefrom is directed in consonance with the discharges of the burners with reference to the '11 imaginary cylinder, each nozzle also being constructed to direct its efilux of recycled gases in a direction divergent with respect to the direction of the burning fuel efilux of its associated burner.

Jackson Dec. 17, 1929 Lucke May 31, 1932 12 Lundgren Sept. 3, 1935 De Baufre Jan. 28, 1941 Kreisinger et al Nov. 28, 1944 Lacerenza Mar. 25, 1952 Keller Nov. 11, 1952 Caracristi Aug. 3, 1954 FOREIGN PATENTS Belgium June 30, 1951 

